In recent years, the range of applications for magnetic recording devices such as magnetic disk devices, flexible disk devices and magnetic tape devices has expanded enormously, and not only has the importance of such devices increased, but the recording density of the magnetic recording media used in these devices has continued to increase markedly. Since the introduction of MR (Magneto Resistance) heads and PRML techniques, the increase in areal recording densities has become even more dramatic. Furthermore, GMR (Giant Magneto Resistance) heads and TuMR (Tunneling Magneto Resistive) heads and the like have been introduced recently, and surface recording densities continue to increase at a pace of 30 to 40% per year.
In this manner, there are strong demands for even higher recording densities for magnetic recording media, and meeting these demands requires further improvements in the coercive force and signal to noise ratio (SNR) of the magnetic recording layer, and higher levels of resolution. In longitudinal magnetic recording systems, which have been widely used until now, as the linear recording density is increased, a self-demagnetizing effect that causes adjacent recording domains of a magnetization transition region to undergo a mutual weakening of magnetization tends to become dominant. In order to avoid this problem, it has been necessary to make the magnetic recording layer progressively thinner, thereby increasing the shape magnetic anisotropy.
On the other hand, as the thickness of the magnetic recording layer is reduced, the size of the energy barrier required to retain the magnetic domain and the size of the thermal energy approach the same level, and therefore, the phenomenon wherein the amount of recorded magnetization is moderated due to the effects of temperature (the thermal fluctuation phenomenon) can no longer be ignored. It is believed that the limit for linear recording density is determined by these types of factors.
Against this background, the use of AFC (Anti Ferromagnetic Coupling) media has recently been proposed as a technique for satisfying the demands for further improvements in the linear recording density of longitudinal magnetic recording systems. In this way, strenuous efforts are being made to avoid the problem of thermal magnetic relaxation, which tends to be a problem in longitudinal magnetic recording.
One powerful technique that is garnering much attention for its potential to enable further increases in areal recording density is the perpendicular magnetic recording technique. In conventional longitudinal magnetic recording systems, the medium is magnetized in the in-plane direction, namely, in a horizontal direction parallel to the surface of the medium, whereas in perpendicular magnetic recording systems, the medium is magnetized in a direction perpendicular to the medium surface. As a result, it is thought that the effects of the self-demagnetizing effect that represents an obstacle to achieving higher linear recording densities in longitudinal magnetic recording systems can be avoided in perpendicular magnetic recording systems, making such perpendicular magnetic recording systems ideal for high density recording. Further, it is also thought that because a certain magnetic layer thickness can be maintained, the effect of thermal magnetic relaxation, which is a significant problem in longitudinal magnetic recording, should be comparatively small.
A perpendicular magnetic recording medium is typically prepared by sequentially providing a seed layer, an intermediate layer, a magnetic recording layer and a protective layer on top of a non-magnetic substrate. Further, after forming the protective layer, a lubricant layer is often applied to the surface of the protective layer. Furthermore, in many cases, a magnetic film known as a soft under layer (SUL) is provided beneath the seed layer.
The intermediate layer is formed for the purpose of further enhancing the properties of the magnetic recording layer. The seed layer controls the crystal orientation of the intermediate layer and the magnetic recording layer, and is said to also have the function of controlling the shape of the magnetic crystals.
In order to produce a perpendicular magnetic recording medium having superior properties, improving the crystal orientation of the magnetic recording layer and reducing the crystal grain size are important factors. In many perpendicular magnetic recording media, a Co alloy material is used for the magnetic recording layer, and the crystal structure adopts a hexagonal close-packed structure. It is important that the (002) crystal plane of the hexagonal close-packed structure is parallel to the substrate surface. In other words, it is important that the crystal c-axis ([002] axis) is aligned along the perpendicular direction with as little disorder as possible.
In order to form crystals of the magnetic recording layer with minimal disorder, Ru has been frequently used as the intermediate layer for the perpendicular magnetic recording medium as it adopts the same hexagonal close-packed structure as conventional magnetic recording layers. Crystals of the magnetic recording layer undergo epitaxial growth on the Ru (002) crystal plane, and therefore, a perpendicular magnetic recording medium having favorable crystal orientation can be obtained (for example, see Patent Document 1).
In other words, improving the degree of orientation of the (002) crystal plane of the Ru intermediate layer also improves the orientation of the magnetic recording layer. Accordingly, improving the recording density of the perpendicular magnetic recording medium requires an improvement in the Ru (002) plane. However, if the Ru is provided directly on top of the amorphous soft under layer, then a thick film is required to obtain superior crystal orientation, and as a result, the non-magnetic Ru weakens the pull of the flux from the head on the soft magnetic material of the soft under layer. Accordingly, conventionally a seed layer oriented in the (111) crystal plane of a face-centered cubic structure has been inserted between the soft under layer and the Ru intermediate layer (for example, see Patent Document 2). The seed layer having a face-centered cubic structure yields a high degree of crystal orientation even with a thin film of approximately 5 (nm), and a Ru layer formed on the seed layer having the face-centered cubic structure has a high degree of crystal orientation even if the layer is thinner than a Ru layer deposited directly on the soft under layer.
Even with a seed layer having the face-centered cubic structure described above, in a conventional seed layer, a single Ru crystal grain of the intermediate layer undergoes epitaxial growth on top of a single crystal grain of the seed layer. Accordingly, one possible method for reducing the crystal grain size of the magnetic recording layer or the crystal grain size of the Ru of the intermediate layer involves reducing the crystal grain size of the seed layer. However, although numerous investigations have been conducted into reducing the crystal grain size, both in terms of the materials used for the seed layer and the intermediate layer, and the methods used for forming these layers, a technique for reducing the crystal grain size of the seed layer while maintaining favorable crystal orientation properties for the intermediate layer and the magnetic recording layer has not been realized yet.
One other reported method of reducing the crystal grain size of the intermediate layer and magnetic recording layer formed on the seed layer is the type of method typically employed for magnetic recording layers, namely, a method that employs a granular structure within the intermediate layer composed of crystal grain portions of Ru or the like, and grain boundary portions of an oxide or the like that surround the crystal grain portions (for example, see non-patent document 1). With this method, by increasing the amount of the oxide and thickening the grain boundary portion of the intermediate layer, the crystal grain size can be reduced by a corresponding amount. Moreover, if an oxide magnetic layer such as CoCrPt—SiO2 is formed on top of the intermediate layer, then the granular structure continues from the intermediate layer through to the magnetic recording layer, which promotes a reduction in the grain size of the magnetic crystal grains and segregation of the oxide, and can be expected to yield reduced noise, resulting in improved recording and reproduction properties.
However, this conversion of the intermediate layer to a granular structure does not represent a reduction in the grain size of the seed layer. Accordingly, the number of crystal grains per unit of surface area does not change. As a result, when future increases are made in the recording density, the number of magnetic crystal grains within a single bit will decrease, causing a reduction in the signal. Furthermore, increasing the relative proportion of the oxide grain boundary portions may cause oxidation of the Co of the magnetic crystal grains, resulting in a further reduction in the signal. Improving recording and reproduction properties to cope with the increase of recording densities requires not only a simple reduction in the grain size of the magnetic crystal grains, but also an increase in the number of crystal grains, namely, an increase in the density of the magnetic crystal grains.
Patent Document 3 discloses a method that uses Mg or Ti that adopts a hexagonal close-packed structure as the seed layer material, and then utilizes the poor wettability between these materials and the Ru used as the intermediate layer material to enable the average grain size of the Ru of the intermediate layer to be reduced to approximately 8 (nm) while maintaining favorable crystal orientation. However, as the thickness of the intermediate layer is increased, the reduced size Ru crystals formed on the seed layer including Mg or Ti tend to readily coalesce with surrounding Ru crystals. As a result, each single magnetic crystal grain tends to undergo epitaxial growth not on a single Ru crystal grain within the intermediate layer, but rather on a plurality of coalesced Ru crystal grains. Because this means that there is no effective reduction in the size of the magnetic crystal grains, the expected dramatic improvements in the recording and reproduction properties tend not to be observed.
Achieving further improvements in the recording and reproduction properties requires a perpendicular magnetic recording medium having excellent recording and reproduction properties, which combines a reduced crystal grain size for the magnetic crystal grains with increased density, while also maintaining or improving the perpendicular orientation properties for the magnetic recording layer. A perpendicular magnetic recording medium that is able to resolve the above issues and is also able to be produced easily has been keenly sought.    [Patent Document 1]    Japanese Unexamined Patent Application, First Publication No. 2001-6158    [Patent Document 2]    Japanese Unexamined Patent Application, First Publication No. 2005-190517    [Patent Document 3]    Japanese Unexamined Patent Application, First Publication No. 2006-155865    [Non-Patent Document 1]    Applied Physics Letters, vol. 89, page 162504.